Process for devolatilizing natural gas liquids

ABSTRACT

A natural gas liquid which has been cryogenically extracted from a natural gas feed is devolatilized by removing a portion of the ethane from the liquid. The natural gas feed stream is initially split into two streams, the smaller part of the stream being fed to the bottom of a demethanizing absorber at a low temperature and the larger part of the stream being fed to the top of the absorber at a substantially lower cryogenic temperature. A methane-rich gas product is recovered from the top of the absorber while the methane-lean liquid in the bottom of the absorber is fed to a non-cryogenic deethanizer. A portion of the volatiles in the methane-lean liquid are removed in the deethanizer and recycled back to the absorber leaving a devolatilized natural gas liquid product.

DESCRIPTION

1. Technical Field

The invention is a process for devolatilizing natural gas condensatesand more specifically a process for partially deethanizing natural gasliquids which have been cryogenically extracted from a natural gas feed.

2. Background Art

Hydrocarbon-containing gas, known generally as natural gas, is producedfrom natural gas wells, oil wells or hydrocarbon refining processes.Natural gas contains a number of combustible hydrocarbon andnon-combustible inorganic constituents which have a broad range ofmolecular weights and boiling points. Natural gas constituents arenormally in a gaseous state at atmospheric conditions of temperature andpressure. However, temperature and pressure vary widely from atmosphericconditions when storing or transporting natural gas causing the heavier,higher boiling point hydrocarbons to condense from the gas to a liquidstate. Many problems accompany the handling of the resulting two-phasecomposition. Condensed natural gas liquids, which impede flow byaccumulating in pipelines and attendant equipment, are a primaryproblem.

Handling of natural gas is simplified, if the lighter gases areseparated from the readily condensable, heavier hydrocarbons so that theheavier hydrocarbons may be stored or transported separately in a liquidstate, i.e. as natural gas liquids. Separation of the natural gasliquids from the lighter gases also enhances the marketability ofspecific natural gas products.

A number of processes have been described in the art for separating thelighter gaseous constituents from the heavier, higher boiling pointhydrocarbons in a natural gas feed. A conventional means for separatingthe natural gas constituents is to pass the gas through an absorptiontower wherein the higher boiling point hydrocarbons are stripped fromthe gas stream upon contact with a liquid absorbant. Such methods areoften more effective when operated cryogenically. U.S. Pat. Nos.4,318,723 to Holmes et al, 4,157,904 to Campbell et al, 3,359,743 toDiNapoli and 3,846,993 to Bates all describe cryogenic separationprocesses whereby the temperature of a natural gas is reduced either byrapid expansion or heat exchange. The resulting condensed liquids areseparated from the gas in a cryogenic column.

U.S. Pat. Nos. 4,285,708 to Politte et al and 4,128,410 to Bacon andGulby, J. G., "Options for Ethane Rejection in the Cryogenic ExpanderPlant," preprint 58th Annual GPA Conv. March 1979, teach cryogenicprocesses, which reduce the amount of ethane in condensed natural gasliquid to minimize the vapor pressure of the liquid. Bacon and Politteet al are staged processes. Bacon cools the gas by heat exchange with arefrigerant to condense the higher boiling point hydrocarbons. Thecooled stream is fed to a separator where the uncondensed gas is removedas pipeline gas. The condensed hydrocarbons are fed to a fractionatingtower to remove ethane from the condensate before recovering thecondensate as liquid product. Politte et al deethanizes a natural gasfeed by splitting it and feeding one stream directly to a deethanizerwhile feeding the other stream to a stabilizer to remove the heaviercomponents as liquids. The overhead vapor from the stabilizer is fed tothe deethanizer to complete the separation of liquids and gases therein.

Non-cryogenic processes for separating readily condensable, heavier,higher boiling point hydrocarbons from a natural gas feed generally donot produce a sufficiently devolatilized natural gas liquid. The vaporpressure of the natural gas liquid is too high to safely store ortransport the liquid by conventional means. The above-cited cryogenicprocesses more effectively separate the gases and liquids in a naturalgas feed to produce a less volatile liquid. However, the substantialadditional cost of cryogenically designed equipment and energy requiredto operate the equipment offset the advantage of these cryogenicprocesses.

A process is needed to separate the heavier natural gas liquids from thelighter gases in a natural gas feed. More specifically, a process isneeded, which sufficiently reduces the vapor pressure of the natural gasliquid by removing a portion of the ethane therefrom to allow safehandling of the liquid. A cost-effective process is needed, which can beretrofitted to existing natural gas separation processes and isadaptable to a broad range of natural gas feed compositions.

DISCLOSURE OF THE INVENTION

The present invention is a process for separating a natural gas feedinto heavier, higher boiling point hydrocarbons and lighter gases bycondensing the natural gas liquid, separating the gas and liquid phases,and removing a portion of the ethane and substantially all of themethane from the liquid product. The natural gas feed may be obtainedfrom any natural gas source including oil and gas production wells orpetroleum refining processes. The natural gas feed contains: (1)combustible, lighter, hydrocarbon gases, such as methane, (2) heavierhydrocarbon constituents, such as ethane, propane, butane, pentane,etc., and (3) non-combustible inorganic gases such as nitrogen, carbondioxide, helium, etc. The natural gas liquid produced according to theinstant process is sufficiently devolatilized to enable safetransportation and storage of it by conventional means.

Basically the process employs two distinct separation unit operationswith intervening heat transfer and expansion apparatus. The lightergases are separated from the heavier hydrocarbons in the first unit, acryogenic demethanizing absorber. The gas product is taken off the topof the absorber and discharged to a pipeline. The liquids are withdrawnfrom the bottom of the absorber and fed to the second unit, anon-cryogenic product deethanizer wherein a portion of the ethane isseparated from the liquid product. The partially deethanized natural gasliquid is withdrawn from the bottom of the deethanizer in adevolatilized condition for transporting. The overhead gases from thedeethanizer are recycled to the demethanizing absorber.

The sizing of the cryogenic demethanizing absorber is minimized byphysically splitting the natural gas feed stream at high pressure andnear ambient atmospheric temperature into a first and a second stream oflike composition before treating the feed. The first stream is fed intothe upper portion of the absorber as a two-phase mixture after itstemperature and pressure are substantially reduced by heat exchange andexpansion. The gas phase is comprised primarily of the lighterconstituents (i.e. methane and non-combustible constituents) and someresidual heavier hydrocarbons. The liquid phase is comprised of anyremaining condensed lighter constituents and substantially all of theheavier hydrocarbons found in the first stream. The second stream,combined with the recycled deethanizer off-gas, is fed into the lowerportion of the demethanizing absorber as a two-phase mixture at atemperature substantially above the temperature of the first stream. Asa result, the gas phase contains more heavier hydrocarbon and the liquidphase contains fewer lighter constituents than those of the firststream.

Essentially four distinct streams are fed to the absorber, a gas andliquid stream to the upper portion and a gas and liquid stream to thelower portion. However, the effective feeds to the absorber are the gasphase of the lower stream and the liquid phase of the upper stream. Theliquid in the lower stream and the gas in the upper stream have littleeffect in the absorber because they are withdrawn in close proximity totheir feed points. Because the two effective feed streams have differentcompositions due to their phase and temperatures differences notedabove, they exchange components on contact. The falling liquid absorbsheavier hydrocarbons in the upflowing gas and the upflowing gas stripslighter constituents from the falling liquid as the two streams flowcountercurrent in the absorber.

A gas product is recovered at the top of the absorber, which containsrelatively few heavier hydrocarbons. The gas is primarily methane and asmaller amount of ethane. The amount of ethane in the gas is regulatedby controlling the operating temperature and pressure of the process.

The liquids are withdrawn from the bottom of the absorber and are usedto cool the second stream before it is fed to the absorber. Thereafter,the liquids are fed to the deethanizer. The liquids accumulated in thebottom of the deethanizer are heated, vaporizing the more volatileportion of the liquid. The less volatile unvaporized liquid is recoveredas liquid product. The vaporized volatile portion flows upward throughthe deethanizer to an overhead condenser. The heavier, less volatilevapors are refluxed and also recovered as liquid product. The lighter,more volatile vapors remain uncondensed and join the second stream tothe absorber as recycle.

The advantages of this process are readily apparent. The processutilizes a deethanizer operated at a non-cryogenic temperature inconjunction with a cryogenic demethanizing absorber. A process, havingonly a non-cryogenic deethanizer, can be retrofitted with ademethanizing absorber to achieve the present process. By splitting thefeed stream, the cryogenic demethanizing absorber can be sized smallerthan one which cryogenically treats the entire unsplit feed stream atonce. The operating conditions of the system, including temperature andpressure, can be varied to treat feed types ranging from lean gas,having a relatively low concentration of natural gas liquids, to richgas, having a relatively high concentration of liquids. These conditionscan also be varied to produce somewhat different residual levels ofheavier hydrocarbons in the gas product and volatiles in the liquidproduct. In any case, the final process products are a pipeline gas fromthe demethanizing absorber containing relatively few readilycondensable, higher boiling point hydrocarbons and a natural gas liquidfrom the deethanizer having only a small amount of volatile hydrocarbonssuch that the vapor pressure of the liquid is sufficiently low to enablesafe handling thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flowsheet of the process described hereincontaining the demethanizing absorber and product deethanizer.

FIG. 2 is a schematic flowsheet of the process modified to maximizeethane recovery in the liquid product.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is a process for separating readily condensable,heavier hydrocarbons from lighter gases in a natural gas feed andpartially deethanizing the condensed heavier hydrocarbons to reducetheir vapor pressure. The initial inlet feed stream to the process is ahydrocarbon-containing gas, generally defined as a natural gas. Thenatural gas contains non-combustible constituents as well as combustiblehydrocarbons. The exact composition of the gas varies depending on itssource. Natural gas sources include natural gas or oil production wellsand hydrocarbon refining processes. Generally the natural gas iscomprised of at least 50 percent methane with the remainder beingheavier (i.e. higher molecular weight) hydrocarbons such as ethane,propane, butane, pentane, etc. and non-combustible inorganicconstituents such as nitrogen and carbon dioxide. The heavierhydrocarbons are readily condensable at high pressure or lowtemperature. The condensed heavier hydrocarbons are natural gas liquids.

Referring to FIG. 1, natural gas feed (10) enters the process at apressure of about 5516 to 8274 kiloPascals absolute (kPaa) and atemperature of about 16° to 49° C. Inlet feed (10) is split into firststream (20) and second stream (30) by any appropriate physical meanssuch that first stream (20) is about 50 to 80 percent of inlet feed(10). The exact fraction of the feed split depends on the composition,pressure and temperature of inlet feed (10). The fraction of the feedsplit may be readily determined for given values of these parameters toobtain a desired degree of ethane recovery.

First stream (20) is cooled in heat exchange means (1) by absorberoutlet gas (40) from demethanizing absorber (100). Cooled stream (21) isthen expanded across expansion means (5) to further reduce itstemperature to a cryogenic level and correspondingly reduce itspressure. Cryogenic temperatures, as defined herein, are those belowabout -28.9° C. At cryogenic temperatures, first stream (22) is atwo-phase composition, having a gas and a liquid phase.

Second stream (30) is cooled in heat exchange means (2) by condensedliquids (71) from demethanizing absorber (100). Stream (31) is used as acoolant in overhead condenser (230) wherein vapors (60) from productdeethanizer (200) are condensed. Overhead condenser (230) may bephysically joined to deethanizer (200) or may be separately linked unit.In either case, it has the same function.

Second stream (32) is expanded across expansion means (6) to cool it andreduce its pressure. Second stream (33) is combined with condenseroff-gas (61) to be recycled to the absorber. Resulting combined stream(90) is a two-phase composition, having a pressure substantially equalto that of first stream (22), but a temperature substantially higherthan first stream (22). The recycle and second stream (90) are fed intothe lower portion of the absorber at (120) while the first stream is fedinto the upper portion at (130). The gas in the lower feed flows upwardthrough the absorber, stripping the lighter constituents from theliquid. The liquid in the upper feed flows downward countercurrent tothe gas, absorbing the heavier, higher boiling point hydrocarbons in thegas. The gas reaching the top of the absorber is removed overhead at(140) as product at a low temperature. It is passed through heatexchange means (1) to cool first stream (20) as noted above. Product gas(41) is methane-rich, containing substantially all of the methane andinorganic gases from the inlet natural gas feed. The amount of ethaneretained in the gas product is dependent on the operating conditions ofthe process.

The methane-lean liquid in the bottom of absorber (70) is withdrawn at(110) at a temperature higher than absorber outlet gas (40). Stream (71)is pumped by pumping means (8) at high pressure into heat exchange means(2) where it cools second stream (30) as noted above. Liquid stream (72)is then fed to the middle of deethanizer (201), i.e. at tray (211) belowuppermost tray (210) of the deethanizer. Liquid (72) combines withcondenser reflux and flows downward through the deethanizer. Liquids(80) are withdrawn from the deethanizer at level (204) below lowermosttray (212) and fed to reboiler (4) where they are heated. Heated liquid(81) is reinserted into the lower portion of the deethanizer at (205).The more volatile portion of liquid (81) is vaporized in the lowerportion of deethanizer (220) and the resulting deethanizer vapor flowsthrough deethanizer (200) countercurrent to the refluxed liquids. Thedeethanizer vapor passes through deethanizer distillation trays, (210),(211) and (212) and into the overhead condenser (230). The more volatilevapor components, which include methane and a portion of the ethane,remain in the vapor phase while the less volatile, heavier hydrocarbonvapor components are refluxed back into the deethanizer. The uncondensedvapor (60) is removed from the condenser at (203) and expanded acrossexpansion means (7) to reduce its pressure and temperature beforerecycling vapor stream (61) to the bottom of the absorber at (120) incombination with second stream (33).

Liquid product (50) is withdrawn from the bottom of the deethanizer at(206) and cooled in heat exchange means (3). The natural gas liquidproduct (51) contains a portion of the ethane and substantially all ofthe heavier, higher boiling point hydrocarbons from the inlet naturalgas feed, i.e. propane, butane, pentane, etc., and substantially none ofthe lighter constituents, i.e. methane, nitrogen, and carbon dioxide.The ethane not in gas product (41) is found in liquid product (51). Asnoted above, the amount of ethane in this stream is dependent on thecomposition of the feed gas and the operating conditions of the system.The temperature and pressure may be varied to achieve the desired degreeof deethanization of the liquid product. In this manner, the process maybe adapted to produce liquids having a range of vapor pressures.

The above-described process, as shown in FIG. 1, minimizes ethanerecovery in the liquid product. Although ethane recovery can be somewhatincreased merely by changing the operating conditions of the process,ethane recovery can be maximized by a slight modification of the processflowsheet, as shown in FIG. 2. Referring to FIG. 2, auxiliaryrefrigerant (82), such as propane, is provided as a coolant in overheadcondenser (230), exiting via stream (83). Second stream (30) bypassesoverhead condenser (230) and is fed into the bottom of absorber (100) at(120) after second stream (30) is first cooled in heat exchange means(2) and combined with recycle (61) to form stream (90).

The following examples are particular applications of the process. Theexamples illustrate how different levels of ethane recovery can beachieved in the liquid product and gas product streams by varying theprocess conditions. Example 1 maximizes ethane recovery in the liquidproduct, using the process of FIG. 2, while Example 2 minimizes ethanerecovery in the liquid product using the process of FIG. 1. Example 3achieves an intermediate level of ethane recovery in the liquid productusing the process of FIG. 1. The examples are not to be construed aslimiting the scope of the present invention.

EXAMPLE 1

A raw natural gas from a wellhead is fed into the system of FIG. 2 at apressure of 7205 kPaa and temperature of 48.9° C. The inlet feed issplit into a first and second stream. The first stream, 72% of the inletfeedstream, is fed to a heat exchanger where it is cooled by theabsorber outlet gas to a temperature of -58.9° C. It is flashed acrossan expansion valve to a temperature of -109° C. and a pressure of 1413kPaa. It is then fed into the demethanizing absorber.

The second stream, 28%, of the inlet feedstream, is fed to a heatexchanger where it is cooled by the liquid from the absorber bottom to atemperature of -40° C. The second stream is then flashed across anexpansion valve and merged with the recycle from the productdeethanizer. The combined stream is fed into the bottom of thedemethanizing absorber at a temperature of -72.8° C. and a pressure of1427 kPaa. The gas product withdrawn overhead from the demethanizingabsorber has a temperature of -108° C. and a pressure of 1412 kPaa. Thegas product is used to cool the first stream, raising the gas producttemperature to 43.3° C. This gas is suitable for pipeline transport.

The phase and composition of the demethanizing absorber inlet and outletstreams are given below in Table 1.

                  TABLE 1                                                         ______________________________________                                        Demethanizing Absorber Streams (Compositions in Mole %)                               Top Feed Bottom Feed                                                                              Gas      Raw                                      Component Liquid  Gas    Liquid                                                                              Gas  Product                                                                              Liquid                             ______________________________________                                        Nitrogen  0.24    2.67   0.08  1.95 2.29   0.09                               Carbon Dioxide                                                                          0.09    0.01   0.10  0.07 0.03   0.18                               Methane   69.96   96.69  23.32 89.31                                                                              96.91  26.79                              Ethane    15.85   0.61   40.47 8.31 0.75   47.34                              Propane   5.46    0.01   12.44 0.30 0.02   10.17                              i-Butane  1.82    0.0    4.89  0.03 0.0    3.34                               n-Butane  3.13    0.0    8.70  0.03 0.0    5.74                               i-Pentane 0.79    0.0    2.28  0.0  0.0    1.46                               n-Pentane 1.50    0.0    4.31  0.0  0.0    2.75                               n-Heptane 1.17    0.0    3.39  0.0  0.0    2.14                               % of Feed 21.40   78.60  6.19  93.81                                          ______________________________________                                    

The raw liquid is withdrawn from the absorber bottom at a temperature of-78.3° C. and is used to cool the second stream in a heat exchanger. Theheat exchanger raises the temperature of the raw liquid to 15° C. at apressure of 2758 kPaa. The liquid is fed into tray 9 of the deethanizer,which contains 13 trays. The liquid passes down to tray 1 where it iswithdrawn, fed to a reboiler and heated to 32.2° C. at a pressure of2758 kPaa. The resulting heated liquid and vapors are fed to the bottomof the deethanizer. The vapors pass up through the deethanizer into theoverhead condenser some of which produce a condensate, refluxing backdown through the deethanizer. The remaining uncondensed vapors arewithdrawn from the condenser at a temperature of -28.9° C. at a pressureof 2723 kPaa. This gas is flashed across an expansion valve to apressure of 1477 kPaa and combined with the second stream to be recycledinto the absorber. The heated liquid not vaporized in the reboiler andreflux are withdrawn from the bottom of the deethanizer. The liquidproduct has a vapor pressure of 2903 kPag at 37.8° C. representing amaximum ethane recovery for the process and the given feed composition.The liquid product is in a condition for storing, transporting orfurther separation if desired.

The compositions of the initial natural gas inlet feed to the process,final gas product from the absorber, and final liquid product from thedeethanizer expressed in mole % are given below in Table 2.

                  TABLE 2                                                         ______________________________________                                        Component  Gas Feed  Gas Product                                                                              Liquid Product                                ______________________________________                                        Nitrogen   2.15      2.29       0.0                                           Carbon Dioxide                                                                           0.03      0.03       0.08                                          Methane    90.97     96.91      2.39                                          Ethane     3.87      0.75       50.37                                         Propane    1.18      0.02       18.53                                         i-Butane   0.39      0.0        6.20                                          n-Butane   0.67      0.0        10.65                                         i-Pentane  0.17      0.0        2.71                                          n-Pentane  0.32      0.0        5.09                                          n-Heptane  0.25      0.0        3.98                                          ______________________________________                                    

EXAMPLE 2

A raw natural gas from a wellhead is fed into the system of FIG. 1 at apressure of 7205 kPaa and a temperature of 48.9° C. The inlet feed issplit into a first and second stream. The first stream, 75% of the inletfeedstream, is fed to a heat exchanger where it is cooled by theabsorber outlet gas to a temperature of -54.4° C. It is flashed acrossan expansion valve to a temperature of -104° C. at a pressure of 1413kPaa. It is then fed into the demethanizer absorber.

The second stream, 25% of the inlet feedstream, is fed to a heatexchanger where it is cooled by the liquid from the absorber bottom to atemperature of 10.6° C. The second stream is then fed to the overheadcondenser of the deethanizer and heated to a temperature of 23.3° C. Itis flashed across an expansion valve and merged with the recycle fromthe product deethanizer. The combined stream is fed into the bottom ofthe demethanizing absorber at a temperature of -4.44° C. and a pressureof 1427 kPaa. The gas product withdrawn overhead from the demethanizingabsorber has a temperature of -92.8° C. and a pressure of 1413 kPaa. Thegas product is used to cool the first stream, raising the gas producttemperature to 43.3° C. This gas is suitable for pipeline transport.

The phase and composition of the demethanizing absorber inlet and outletstreams are given below in Table 3.

                  TABLE 3                                                         ______________________________________                                        Demethanizing Absorber Streams (Compositions in Mole %)                               Top Feed Bottom Feed                                                                              Gas      Raw                                      Component Liquid  Gas    Liquid                                                                              Gas  Product                                                                              Liquid                             ______________________________________                                        Nitrogen  0.17    2.47   0.04  2.03 2.22   0.06                               Carbon Dioxide                                                                          0.10    0.02   0.01  0.03 0.03   0.02                               Methane   56.94   96.42  8.51  87.67                                                                              93.75  12.01                              Ethane    21.86   1.06   4.60  6.60 3.92   18.57                              Propane   8.33    0.03   6.32  2.17 0.08   30.08                              i-Butane  2.75    0.0    3.12  0.39 0.0    8.59                               n-Butane  4.70    0.0    7.61  0.63 0.0    14.61                              i-Pentane 1.19    0.0    4.88  0.15 0.0    3.69                               n-Pentane 2.23    0.0    12.62 0.26 0.0    6.94                               n-Heptane 1.74    0.0    52.28 0.08 0.0    5.42                               % of Feed 14.32   85.68  0.30  99.70                                          ______________________________________                                    

The raw liquid withdrawn from the absorber bottom at a temperature of-37.2° C. and is used to cool the second stream in a heat exchanger. Theheat exchanger raises the temperature of the raw liquid to 43.3° C. at apressure of 2758 kPaa. The liquid is fed into tray 9 of the deethanizer,which contains 13 trays. The liquid passes down to tray 1 where it iswithdrawn, fed to a reboiler and heated to 113° C. at a pressure of 2758kPaa. The resulting heated liquid and vapors are fed to the bottom ofthe deethanizer. The vapors pass up through the deethanizer into theoverhead condenser some of which produce a condensate, refluxing backdown through the deethanizer. The remaining uncondensed vapors arewithdrawn from the condenser at a temperature of 13.3° C. at a pressureof 2723 kPaa. This gas is flashed across an expansion valve to apressure of 1427 kPaa and combined with the second stream to be recycledinto the absorber. The heated liquid not vaporized to the reboiler andreflux are withdrawn from the bottom of the deethanizer and cooled to48.9° C. The liquid product has a vapor pressure of 607 kPag at 37.8° C.representing a minimum ethane recovery for the process and the givenfeed composition. The liquid product is in a condition for storing,transporting or further separation if desired.

The compositions of the initial natural gas inlet feed to the process,final gas products from the absorber, and final liquid product from thedeethanizer expressed in mole % are given below in Table 4.

                  TABLE 4                                                         ______________________________________                                        Component  Gas Feed  Gas Product                                                                              Liquid Product                                ______________________________________                                        Nitrogen   2.15      2.22       0.0                                           Carbon Dioxide                                                                           0.03      0.03       0.0                                           Methane    90.97     93.75      0.0                                           Ethane     3.87      3.92       2.24                                          Propane    1.18      0.08       37.15                                         i-Butane   0.39      0.0        13.09                                         n-Butane   0.67      0.0        22.57                                         i-Pentane  0.17      0.0        5.73                                          n-Pentane  0.32      0.0        10.79                                         n-Heptane  0.25      0.0        8.43                                          ______________________________________                                    

EXAMPLE 3

A raw natural gas from a wellhead is fed into the system of FIG. 1 at apressure of 7205 kPaa and a temperature of 48.9° C. The inlet feed issplit into a first and second stream. The first stream, 75% of the inletfeed stream, is fed to a heat exchanger where it is cooled by theabsorber outlet gas to a temperature of -55.6° C. It is flashed acrossan expansion valve to a temperature of -105° C. and a pressure of 1413kPaa. It is then fed into the demethanizing absorber.

The second stream, 25% of the inlet feed stream, is fed to a heatexchanger where it is cooled by the liquid from the absorber bottom to atemperature of -18.9° C. The second stream is then fed to the overheadcondenser of the deethanizer and raised to a temperature of -2.78° C. Itis flashed across an expansion valve and merged with the recycle fromthe product deethanizer. The combined stream is fed into the bottom ofthe demethanizing absorber at a temperature of -35° C. and a pressure of1427 kPaa. The gas product withdrawn overhead from the demethanizingabsorber has a temperature of -97.2° C. and a pressure of 1413 kPaa. Thegas product is used to cool the first stream, raising the gas producttemperature to 43.3° C. This gas is suitable for pipeline transport.

The phase and composition of the demethanizing absorber inlet and outletstreams are given below in Table 5.

                  TABLE 5                                                         ______________________________________                                        Demethanizing Absorber Streams (Compositions in Mole %)                               Top Feed Bottom Feed                                                                              Gas      Raw                                      Component Liquid  Gas    Liquid                                                                              Gas  Product                                                                              Liquid                             ______________________________________                                        Nitrogen  0.18    2.49   0.05  2.00 2.25   0.07                               Carbon Dioxide                                                                          0.10    0.02   0.02  .03  0.03   0.03                               Methane   58.87   96.50  11.88 87.65                                                                              94.94  16.52                              Ethane    20.83   0.97   11.59 8.22 2.72   38.31                              Propane   7.88    0.03   9.95  1.29 0.06   18.15                              i-Butane  2.64    0.0    7.04  0.27 0.0    5.82                               n-Butane  4.52    0.0    16.96 0.40 0.0    10.02                              i-Pentane 1.15    0.0    7.84  0.06 0.0    2.54                               n-Pentane 2.15    0.0    16.93 0.08 0.0    4.79                               n-Heptane 1.68    0.0    17.73 0.0  0.0    3.74                               % of Feed 14.85   85.15  1.27  98.73                                          ______________________________________                                    

The raw liquid is withdrawn from the absorber bottom at a temperature of-56.7° C. and is used to cool the second stream in a heat exchanger. Theheat exchanger raises the temperature of the raw liquid to 43.3° C. at apressure of 2758 kPaa. The liquid is fed into tray 9 of the deethanizer,which contains 13 trays. The liquid passes down to tray 1 where it iswithdrawn, fed to a reboiler and heated to 60.0° C. at a pressure of2758 kPaa. The resulting heated liquid and vapors are fed to the bottomof the deethanizer. The vapors pass up through the deethanizer into theoverhead condenser some of which produce a condensate, refluxing backdown through the deethanizer. The remaining uncondensed vapors arewithdrawn from the condenser at a temperature of -13.3° C. and apressure of 2723 kPaa. This gas is flashed across an expansion valve toa pressure of 1427 kPaa and combined with the second stream to berecycled into the absorber. The heated liquid not vaporized in thereboiler and reflux are withdrawn from the bottom of the deethanizer andcooled to 48.9° C. The liquid product has a vapor pressure of 1875 kPagat 37.8° C. The liquid product is in a condition for storing,transporting or further separation if desired.

The compositions of the initial natural gas inlet feed to the process,final gas product from the absorber, and final liquid product from thedeethanizer expressed in mole % are given below in Table 6.

                  TABLE 6                                                         ______________________________________                                        Component  Gas Feed  Gas Product                                                                              Liquid Product                                ______________________________________                                        Nitrogen   2.15      2.25       0.0                                           Carbon Dioxide                                                                           0.03      0.03       0.01                                          Methane    90.97     94.94      1.46                                          Ethane     3.87      2.72       29.72                                         Propane    1.18      0.06       26.50                                         i-Butane   0.39      0.0        9.14                                          n-Butane   0.67      0.0        15.75                                         i-Pentane  0.17      0.0        4.00                                          n-Pentane  0.32      0.0        7.53                                          n-Heptane  0.25      0.0        5.89                                          ______________________________________                                    

The three examples provided above all treat a natural gas feed streamhaving a common, relatively lean composition. It is apparent that thepresent process may be used to treat richer or leaner natural gas feedsby changing such process operating conditions as temperature, pressureand fractional feed division. Where the amount of liquids in the naturalgas feed exceeds the capacity of the system as in a rich natural gas,the initial natural gas feed can be treated by additional refrigerationat the front end of the process before the feed is split to remove someof the liquids.

While the foregoing embodiment of the invention has been described andshown, it is understood that all alternatives and modifications, such asthose suggested, and others may be made thereto, and fall within thescope of the invention.

I claim:
 1. A process comprising:retrofitting an existing natural gasseparation process to obtain a retrofitted process for separating anatural gas feed selected from a broad range of compositions into amethane-rich gas product and a partially deethanized natural gas liquidproduct, said retrofitted process including the steps of: (a) dividingsaid natural gas feed into a first stream and a second stream, the twostreams having the same composition; (b) reducing the temperature andpressure of said first and second streams such that each said stream hasa distinct liquid phase and a distinct gas phase, said first stream hasa temperature substantially lower than said second stream, and at leastsaid first stream has a cryogenic temperature; (c) feeding said firststream into the upper portion of a demethanizing absorption means andsaid second stream into the lower portion of said absorption means; (d)recovering said methane-rich gas product from the top of said absorptionmeans and a methane-lean natural gas liquid from the bottom of saidabsorption means; (e) feeding said methane-lean natural gas liquid intoa deethanizer means wherein an uncondensed vapor comprised of a portionof the ethane and substantially all of the methane in said methane-leannatural gas liquid is separated from said methane-lean natural gasliquid at a non-cryogenic temperature; (f) recovering said uncondensedvapor from the top of said deethanizer means and recycling said vaporinto said second stream of step (c) prior to feeding said second streaminto said absorption means; and (g) recovering said partiallydeethanized natural gas liquid product from the bottom of saiddeethanizer means comprised of the remaining portion of ethane andheavier hydrocarbons in said methane-lean natural gas liquid and whichis less volatile than said methane-lean natural gas liquid.
 2. Theprocess of claim 1 wherein said methane-lean natural gas liquid coolssaid second stream in a heat exchange means to initiate the temperaturereduction of step (b) prior to feeding said natural gas liquid to saiddeethanizer means.
 3. The process of claim 1 wherein the temperature andpressure of said first and second streams are reduced a predeterminedamount to achieve a predetermined level of ethane recovery in saidmethane-rich gas product and natural gas liquid product.
 4. The processof claim 1 wherein the relative fraction of said natural gas feeddivided into said first and second streams is predetermined to achieve apredetermined level of ethane recovery in said methane-rich gas productand natural gas liquid product.
 5. The process of claim 4 wherein about50 to about 80% of said natural gas feed is divided into said firststream and about 50 to about 20% of said natural gas feed is dividedinto said second stream.
 6. The process of claim 1 wherein the pressureof said natural gas feed is about 5516 to about 8274 kPaa and thetemperature of said natural gas feed is about 16° to about 49° C.
 7. Theprocess of claim 1 wherein said deethanizer means is comprised of acolumn, having a plurality of vertically spaced distillation trays, anda condenser.
 8. The process of claim 7 wherein said methane-lean naturalgas liquid is fed to said deethanizer means at an intermediate point insaid column between the uppermost and lowermost trays.
 9. The process ofclaim 8 wherein said methane-lean natural gas liquid, having passedbelow said lowermost tray, is sufficiently heated to vaporize the morevolatile portion of said natural gas liquid into a deethanizer vapor andthe remaining less volatile unvaporized portion of said natural gasliquid recovered as said partially deethanized natural gas liquidproduct.
 10. The process of claim 9 wherein said deethanizer vapor ispassed through said column into said condenser where the less volatileportion of said deethanizer vapor is refluxed and recovered as partiallydeethanized natural gas liquid product and the more volatile portion ofsaid deethanizer vapor not refluxed is recycled to said absorption meansas said uncondensed vapor.
 11. The process of claim 9 wherein saidnatural gas liquid is heated in a reboiler.
 12. The process of claim 7wherein said cooled second stream is circulated through said condenseras a coolant prior to recycling said uncondensed vapor into said secondstream.
 13. The process of claim 7 wherein ethane recovery in saidnatural gas liquid product is maximized by circulating an auxiliaryrefrigerant through said condenser as a coolant.
 14. The process ofclaim 1 wherein said methane-rich gas product cools said first stream ina heat exchange means to initiate the temperature reduction of step (b).15. The process of claim 1 wherein said first stream is expanded acrossa first expansion means to effect the temperature and pressure reductionof step (b).
 16. The process of claim 1 wherein said second stream isexpanded across an expansion means to effect the temperature andpressure reduction of step (b).